I Field of the Invention
The present invention relates generally to satellite communication systems and networks. More particularly, the present invention relates to determining the location of mobile stations relative to satellites, with higher accuracy.
II Related Art
A typical satellite-based communications system comprises at least one terrestrial base station (hereinafter referred to as a gateway), at least one user terminal (for example, a mobile telephone), and at least one satellite for relaying communications signals between the gateway and the user terminal. The gateway provides links from a user terminal to other user terminals or communications systems, such as a terrestrial telephone system.
Mobile user terminals have enabled users to take their communication capabilities wherever they go. This imposes a burden on a satellite based communications system carrier of determining the location of the user terminal. The need for user terminal position information stems from several considerations. One consideration is that the system should select an appropriate gateway for providing the communications link. One aspect of this consideration is allocation of a communications link to the proper service provider (for example, a telephone company). A service provider is typically assigned a particular geographic territory, and handles all calls with users in that territory. When communications are required with a particular user terminal, the communications system can allocate the call to a service provider based on the territory within which the user terminal is located. This may include allocation to a gateway servicing that territory. In order to determine the appropriate territory, the communications system requires the position of the user terminal. A similar consideration arises when calls must be allocated to service providers based on political boundaries or contractual services.
Many mobile telephone users of satellite-based communications systems also desire to know their user terminal position. For example, a mobile telephone user, camping in the woods, becomes lost or unsure of a location or distance from a desired destination. In addition to making an emergency phone call for help or direction assistance, the mobile telephone user would like to be provided with user terminal position information. Such information may aid mobile telephone users in finding their own way to either safety or a desired location.
Presently, user terminal position information is obtained for communication system use only. Error specifications for current communication systems require that user terminal position information be determined within an accuracy of ten kilometers. An error of ten kilometers may be sufficient for system operations, but is generally insufficient if user terminal position information is being supplied to a user, especially for emergency purposes. User terminal position information must be provided with sufficient accuracy to be meaningful to the user.
Several methods of determining the location of the user terminal involve measuring the distance (range) between the user terminal and the satellite and the rate of change of that distance (range rate). When these measurements are combined with other data, such as the position of the satellite, the location of the user terminal can be determined. Techniques for determining user terminal location using satellite-user terminal range and range rate are disclosed in commonly-owned, application Serial Nos. 8/723,751, entitled xe2x80x9cPosition Determination Using One Low-Earth Orbit Satellite;xe2x80x9d 08/723,725, entitled xe2x80x9cUnambiguous Position Determination Using Two Low-Earth Orbit Satellites;xe2x80x9d 09/150,500, entitled xe2x80x9cAccurate Range And Range Rate Determination In A Satellite Communication System;xe2x80x9d and 09/150,077, entitled xe2x80x9cFast User Terminal Position Determination In A Satellite Communications System,xe2x80x9d each being incorporated herein by reference.
In the above-described patent applications, the accuracy of determining the distance (range) between one or more satellites and a user terminal is a few kilometers for one satellite positioning, and approximately 500 meters for two satellite positioning. By the year 2001, the Federal Communications Commission (FCC) in the United States has required that the location of a person making an emergency xe2x80x9c911xe2x80x9d call be determined to within 125 meters. Similar requirements may be the subject of governmental action in other countries. In addition, other types of requests for assistance are also generally viewed as being better addressed with improved accuracy in position determination. Some commercial services of a non-emergency nature also desire improved location accuracy.
What is needed is a system and method for allowing a user to make a position call to receive useful user terminal position information. What is further needed is a system and method for obtaining more accurate measurements, which will in turn yield a more accurate determination of the location of a user terminal to within 125 meters to satisfy FCC emergency 911 call, and other, user position requirements.
The present invention satisfies the above mentioned needs by providing an improved system and method for determining the location of a user terminal with a high degree of accuracy. The present invention uses fine positioning to determine the location of a user terminal in a satellite communications system. Fine positioning is defined as higher accuracy positioning. Fine positioning provides an improved accuracy that is of value to the user by repeating N times the basic active positioning measurement sets during a position call, extending measurements based on a change in geometry, and adding a reference user terminal for providing differential correction.
According to the method of the present invention, the user makes a position call. A position call prompts a gateway to establish a traffic channel on which a positioning message will yield the basic set of measurements: round-trip delay and round-trip Doppler in single-satellite positioning, and the addition of delay-difference and Doppler difference to a second satellite in two-satellite positioning. The accumulated measurements are used to determine position and theoretical error. The call is terminated when an accuracy threshold is reached or a time limit has expired. The user terminal receives a position message or a failure message.
Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.